1. Field of the Invention
The present invention relates to the field of optical semiconductor devices including a semiconductor laser device used as a light source for optical information processing, a signal source for optical communications, or an excitation light source for fiber amplifiers, semiconductor optical amplifiers (SOAs), superluminescent diodes (SLDs), and optical modulators. More particularly, the present invention relates to an optical semiconductor device having coating films provided on the end faces of its optical semiconductor element.
2. Description of the Related Art
Description will be made below of a semiconductor laser device, which is one of optical semiconductor devices.
FIG. 53 is a schematic diagram showing an output dependence of the wavelength of a conventional semiconductor laser.
The diagram is disclosed in an article entitled “High-power visible GaAlAs lasers with self-aligned strip buried heterostructure” by Ohtoshi et al., J. Appl. Phys., Vol. 56, No. 9, pp. 2491-2496, 1984.
The output dependence in FIG. 53 is exhibited by a semiconductor laser having an SiO2 film and an SiO2 film/amorphous silicon (hereinafter referred to as a-Si) multilayer film coated on its front and rear end faces, respectively. The reflectance of the front end face is 6%, while that of the rear end face is 94%.
As the optical output increases from 1 mW to 30 mW, the oscillation wavelength increases from 780 nm to 786 nm, showing a change of 6 nm, as shown in FIG. 53. On a per-milliwatt basis, the change is 0.21 nm/mA, which is 0.21 nm/mA assuming that the slope efficiency is 1 mW/mA.
This change in the wavelength is attributed to an increase in the temperature of the active layer due to increased injection current. The magnitude of such a change with the AlGaAs semiconductor laser is said to be approximately 0.2 to 0.3 nm/° C. on a per-temperature basis, while that for the InGaAsP semiconductor laser is said to be approximately 0.4 to 0.7 nm/° C. (see a book entitled “Optical Communications Device Engineering”, second edition, by Hiroo Yonezu, Kougakutosho Ltd., pp. 244-255)
Thus, as shown in FIG. 53, even if the optical output is changed, the oscillation wavelength remains around 780 nm. That is, when the injection current (which corresponds to the optical output) is changed, the oscillation wavelength continuously changes by only approximately 0.21 nm/mA.
Furthermore, since the SiO2 film provided on a front end face of the conventional semiconductor laser has a thickness of only λ/4 (λ denotes the wavelength), the reflectance of the end face is approximately 6%, which is much larger than a desired low reflectance of 1% or less.
Configurations of the nonreflective films of conventional semiconductor lasers are described in, for example, Japanese Patent No. 3014208 and IEE Electronics Lett. Vol. 31, No. 31, pp. 1574-1575.
Thus, the conventional semiconductor laser having the configuration described above can be provided with a low-reflective end face coating film having a reflectance of 6% at lowest.
The conventional semiconductor laser may be provided with a coating film having a total film thickness less than ¼ of a desired wavelength λ0 to set the width of the wavelength region of the film which is a neighborhood of the wavelength λ0 and whose reflectance is 1% or smaller to be wider than 100 nm. In such a configuration, however, since the total film thickness is thin, the heat dissipation is reduced, which may degrade the end faces.
Furthermore, if a coating film is formed such that no reflection occurs at a desired wavelength λ0 and the thickness of the coating film is thicker than ¼ of the wavelength λ0 to increase the heat dissipation, a problem arises that the reflectance dependence on the wavelength is steep.
FIG. 54 is a schematic diagram showing the configuration of a nonreflective film of a conventional semiconductor laser.
The configuration of the nonreflective film shown in FIG. 54 is disclosed in, for example, Japanese Patent No. 3014208 and IEE Electronics Lett. Vol. 31, No. 31, pp. 1574-1575.
In the figure, reference numeral 200 denotes a conventional semiconductor laser; 202 denotes a semiconductor laser element having an effective refractive index of np; and 204 denotes a first layer film with a refractive index of n01 and a film thickness of d01 formed on an end face of the semiconductor laser 202. Reference numeral 206 denotes a second layer film with a refractive index of n02 and a film thickness of d02 formed on a surface of the first layer film 204. Reference numeral 208 denotes a third layer film with a refractive index of n03 and a film thickness of d03 formed on a surface of the second layer film 206. Reference numeral n0 denotes the refractive index of the free space on a surface of the third layer film 208.
FIG. 55 includes graphs each showing the wavelength dependence of the reflectance of a conventional nonreflective film.
In the figure, curves a and b each indicate the wavelength dependence of the reflectance of a nonreflective film near the wavelength λ0 (=1.3 μm) when the effective refractive index (denoted by nc) of the semiconductor laser element 202 is 3.2.
Specifically, the curve a indicates a reflectance obtained when: the first layer film 204 and the third layer film 208 are each formed of Al2O3 having a refractive index (denoted by n01 or n03, respectively) of 1.6; the second layer film 206 is formed of amorphous silicon (a-Si) having a refractive index (denoted by n02) of 3.2; and the film thicknesses d01, d02, and d03 of the above first to third layer films are 90.23 nm, 8.25 nm, and 90.23 nm, respectively.
The curve b indicates a reflectance obtained when: the first layer film 204 and the third layer film 208 are each formed of Al2O3 having a refractive index (denoted by n01 or n03, respectively) of 1.6; the second layer film 206 is formed of amorphous silicon (a-Si) having a refractive index (denoted by n02) of 3.2; and the film thicknesses d01, d02, and d03 of the above first to third layer films are 90.23 nm, 199.43 nm, and 90.23 nm, respectively.
If the effective refractive index nc of the semiconductor laser 202 is 3.2, nf=(nc*n0)1/2=1.78885. Assuming that the wavelength λ0=1.3 μm, λ0/4 is approximately 325 nm.
In the example indicated by the curve a, the total film thickness (n01*d01+n02*d02+n03*d03) of the three layer films is 314.5 nm, which is approximately equal to λ0/4. The low-reflective region whose reflectance is 1% or smaller has a width of 265 nm, which is wide. However, in this case, since it is not always possible to obtain sufficient film thickness, the heat dissipation may be reduced, which might degrade the end faces of the semiconductor laser element 202.
In the example indicated by the curve b, on the other hand, the total film thickness is as thick as approximately 927 nm, increasing the heat conductivity. However, the low-reflective region whose reflectance is 1% or smaller has a width of only 55 nm, which is extremely narrow.
On the other hand, to realize the characteristics of an ideal single layer film, conventional methods use a two-layer film or a three-layer film to form a nonreflective film and increase the film thickness.
For example, Japanese Patent No. 3014208 discloses a nonreflective coating film made of a three-layer film in which the total film thickness (n01*d01+n02*d02+n03*d03) is set at an integer multiple of ¼ of a desired wavelength λ0, where n01, n02, and n03 denote the refractive indexes of the coating films (constituting the three-layer film) whereas d01, d02, and d03 denote their thickness. This configuration makes the characteristic matrix of the three-layer film equal to that of an ideal single-layer film.
In another method which uses a two-layer film, the film thickness (n01*d01) of the first layer and the film thickness (n02*d02) of the second layer are each made equal to ¼ of a desired wavelength λ0, and they are laminated one on the other.
However, the degree of freedom for selecting materials is reduced in the above methods in which the total film thickness (n01*d01+n02*d02+n03*d03) is set at an integer multiple of ¼ of a desired wavelength λ0, or the film thickness (n01*d01) of the first layer and the film thickness (n02*d02) of the second layer are each made equal to ¼ of a desired wavelength λ0, making it difficult to design the device.
It should be noted that Japanese Patent Laid-Open Publication No. Hei 3(1991)-293791 discloses a technique for a semiconductor laser device in which dielectric thin films formed in two or more layers are used as a non-reflective coating film on an end face, wherein the first layer provides a passivation function and the second and subsequent layers are made of a λ/4 non-reflective coating film.